When I got my speeding ticket on the M6 Toll, I was told by the cops in the unmarked police car that followed me that originally they didn't police it as it was a private road, but they had received too many complaints from the public about people driving too fast.
It may have been just spin, but I got the feeling that they thought it was a bit silly. They commented that it was a sunny day, a clear road, a modern car capable of driving at that speed and that I was by no means the fastest, and that it was not a safety issue, but they were obliged to enforce the law.